ER20 collet searches usually ask two different questions: what the collet system is, and which tool diameter a particular ER20 collet can hold. The safe answer is in the manufacturer’s clamping-range table, not in a generic number copied from a different series. This guide explains how to choose an ER20 collet, verify the maximum diameter and buy a set for CNC milling.
What is an ER20 collet?
ER20 identifies the taper and size family of an ER collet chuck. The collet is a slotted spring component that grips a round shank when the nut draws it into the matching taper. A complete toolholding system also includes the holder body, nut, machine interface, tool shank and tightening method.
Haas lists ER series by holder taper size and provides separate ER collet and chuck products. Kennametal publishes ER20 collet entries with a specified maximum clamping diameter and collapse range. Those product tables illustrate why the series name alone cannot answer the “maximum diameter” question.
How to answer “ER20 collet max diameter”
Read the row for the exact collet size. The nominal size is the largest shank diameter that the specified collet is designed to grip; the allowable collapse is the smaller range stated by that manufacturer. Do not force a smaller shank into a collet whose range does not include it, and do not assume every ER20 brand uses the same tolerance or length.
| Check | Evidence to request |
|---|---|
| Collet size | Marking and manufacturer range table for the individual ER20 collet. |
| Holder compatibility | ER20 taper, nut, machine interface and gauge length. |
| Tool shank | Round, clean shank within the stated range; no burrs or flats unless the holder is designed for them. |
| Runout and balance | Supplier specification and inspection method at the working gauge length. |
| Tightening | Correct nut, wrench, torque method and cleaning procedure. |
Choosing an ER20 collet set
An “ER20 collet set” is useful when the shop uses several round-shank tool sizes, but the set should be selected around the actual tools and workholding. List cutter shanks, drills, reamers or probes, then check which sizes need dedicated collets. A set with many unused sizes is less useful than a smaller set with clear markings and a storage method that protects the tapers and slots.
- Confirm the machine-side holder and nut before ordering the collets.
- Match metric or inch tool shanks to the range table instead of relying on a visual fit.
- Keep collets, nut and holder clean; chips on the taper can change seating.
- Use the supplier’s tightening and inspection method.
- Record the tool, holder, gauge length and program revision for repeat jobs.
How ER20 choice affects the CNC process
Toolholding is one link in the machine stiffness chain. A long gauge length, dirty taper, damaged nut or incorrect range can affect runout, tool life and finished surfaces. The collet does not compensate for a spindle, holder, fixture or cutter that is unsuitable for the load. Include the toolholder in the process trial and in the acceptance record.
For CNC machine selection, compare the configured spindle, tooling and workholding with the representative part. CNC Tech’s T745 heavy-duty CNC drill tap center and CNC VMC1160 vertical machining center pages are product starting points; request the current spindle and tooling specification for the actual order. Send the drawing and tool list to CNC Tech for a configuration review.
FAQ
What is the maximum diameter of an ER20 collet?
Use the maximum diameter printed or listed for the exact collet. The ER20 series name alone is not enough; different sizes and standards have different ranges.
Can one ER20 collet grip every small tool?
No. Each collet has a specified range. Select the collet whose published range includes the shank diameter, and use the correct nut and holder.
Is an ER20 set suitable for every CNC spindle?
No. The spindle or toolholder must accept the ER20 chuck or adapter, and the gauge length, balance and speed must suit the machine and process.
Sources: Haas ER collet holder information; Kennametal ER collet specifications; ISO 230-1; and OSHA machine guarding.